In long term evaluation (LTE), at the time of establishing uplink synchronization or at the time of performing hand over, preamble signals are transmitted using RACH, which stands for Random Access CHannel and serves as an upbound channel. Thus, a mobile device at the transmitting side transmits predetermined preamble sequences to the RACH. On the other hand, a base station at the receiving side subjects the preamble sequences, which arrive from the mobile device, and a known data series to correlation calculation so as to obtain a delay profile. Then, the base station makes use of detecting windows to compare peaks of the delay profile with a threshold value and detects whether or not the preamble signals have been transmitted.
In such a preamble detecting method, if the preamble sequences arriving from a mobile device have propagation environment factors such as noise or fading, then the preamble signals may get detected by mistake. More particularly, if the preamble sequences arriving from a mobile device have propagation environment factors such as noise or fading, the peaks of the delay profile undergo changes. Consequently, despite the fact that no preamble signal is actually transmitted by the mobile device, the base station determines that a peak of the delay profile has exceeded the predetermined threshold value at a detecting window and thus mistakenly detects that a preamble signal has been transmitted by the mobile device. In the following explanation, the condition in which a mobile device is mistakenly detected to have transmitted a preamble signal is referred to as “preamble signal false detection”.
In order to avoid the condition of preamble signal false detection, a technology has been disclosed for adjusting the width of a detecting window according to the propagation delay amount between the mobile device and the base station. In such a conventional technology, the width of a detecting window is adjusted so as to reduce interference with the preamble sequence corresponding to an adjacent detecting window. That enables achieving reduction in the occurrence of preamble signal false detection.
Patent Literature 1: International Publication Pamphlet No. 2008/016112
Patent Literature 2: International Publication Pamphlet No. 2008/129797
However, in the conventional technology of adjusting the width of the detecting window, if the mobile device is located either in the vicinity of the base station or at the end portion of a cell housed in the base station, there remains a possibility that the preamble signals are detected by mistake.
That is, if the mobile device is located either in the vicinity of the base station or at the end portion of a cell, sometimes the delay profile reaches a peak near the boundary of a detecting window and the side lobe of the delay profile leaks into the adjacent detecting window. In such a case, in the conventional technology described above, at the time of establishing uplink synchronization or at the time of performing hand over; when a mobile device starts transmitting the preamble sequences, firstly the base station instructs the mobile device to transmit all preamble sequences in order starting from lower sequence numbers to higher sequence numbers. Accordingly, in the conventional technology, the mobile device transmits the preamble sequences corresponding to all detecting windows to the base station. Hence, in the other detecting window into which a side lobe of the delay profile has leaked, it is mistakenly detected that the peak of the delay profile has exceeded a predetermined threshold value. As a result, there remains a possibility that the preamble signals are detected by mistake.